1. Field of the Invention
The present invention relates to slow-release microcapsules containing biologically active compounds. Particularly, the present invention relates to a slow-release microcapsule composition for safe delivery of agriculturally active material and a process for preparing the same.
2. Description of the Related Art
Insecticides, particularly pyrethroids such as lambda cyhalothrin, cypermethrin, fenvalerate, permethrin, alpha-cypermethrin, and similar compounds, which are moderately toxic in the technical form, have a high skin irritant property and in some cases may provoke an adverse skin reaction such as burning, tingling, numbness or prickling sensation, also generally known as paraesthesia. These skin irritant properties are most pronounced in the areas of an operator's face, hand, and neck during application of these insecticides. For delivering such agriculturally active materials to a target, encapsulation or microencapsulation is one of the safest modes. Encapsulation, or more specifically microencapsulation, is increasingly being used to target specific insects, as it also provides a safer handling of toxic fungicides and insecticides. A microcapsule is a 10−3 m to 10−9 m diameter particle, composed of an inner core material and an outer shell.
Microencapsulation of a number of different agriculturally active materials and pesticides has been known for many years by using various processes or techniques for solving other problems such as volatility, UV degradation of the actives, reducing aquatic toxicity, pollution in air, water, soil, and also protecting untargeted insects and the like.
The use of microcapsules for the slow or controlled release of liquid or solid active ingredient is well known in the art. Typically, for slow or controlled release, the walls of the microcapsules are porous in nature releasing the entrapped material to the surrounding medium at a slow or controlled rate by diffusion through the pores of the wall. The controlled-release techniques have improved the efficiency of herbicides, insecticides, bactericides and fertilizers.
Non-agricultural uses of slow-release or controlled release microcapsules include encapsulated dyes, pigments, inks, flavoring agents, fragrances etc.
Typically, microcapsules may be formed by physical methods such as spray coating, spray drying, pan coating, rotary disk atomization and the like; and chemical methods such as phase separation, interfacial polymerization, simple and complex coacervation method and the like. The prior art includes microcapsules produced by interfacial polymerization by forming a capsular wall that involves hydrolysis of an isocyanate monomer to form an amine which in turn reacts with another isocyanate monomer to form the polyurea enclosure polymers by reaction of isocyanate monomers. Capsules capable of controlled release of encapsulated organic active material enclosed in a polyurea capsule are also described.
Generally, release rates of the microcapsules are governed by the capsule particle size, the thickness of the wall and the permeability of the wall. It is also known that release rates can be appreciably varied by altering the chemical composition and thus the permeability of microcapsule walls. For example, the prior art describes microcapsules formed from an aminoplast shell wall and an encapsulated ingredient enclosed within the wall containing a base-cleavable ester moiety. In the presence of base, the capsule walls relatively quickly disintegrate or degrade so as to produce a relatively quick release of the encapsulated materials. If the capsules are not in a basic environment, they function as typical diffusion controlled release microcapsules, permitting release of the encapsulated substance into the surrounding area in a controlled manner which is determined primarily by wall characteristics of the shells such as thickness, capsule size, permeability, etc. If, on the other hand, the capsules are placed in a basic environment, preferably in a situation in which the resulting pH is from about 8 to about 13, preferably from about 9 to about 11, the cross-linking moieties in the capsule wall are cleaved so as to “trigger” or initiate breakdown of the capsule wall. Depending on the conditions of the environment and on the particular structure of the capsule wall, the resulting breakdown may occur relatively quickly or relatively slowly. Additionally, the prior art provides acid-triggered or -sensitive microcapsules which contain encapsulated material that are sensitive to the presence of acid, and on exposure to an acidic environment break down and/or become porous so as to release the encapsulated substance into the surrounding environment. Furthermore, the prior art also describes microcapsule wall compositions containing disulfide units and providing a semi-permeable barrier. The walls are made from materials where the wall forming materials contain sulfur atoms such as polythiol compounds.
The prior arts known for the encapsulation include a polyurea shell wall or urea/formaldehyde shell. However, the capsule shell wall materials are not biodegradable and may not be environmental friendly. Moreover, the isocyanates may cause skin/eye irritation due to its high toxicity.
The co-pending application U.S. Ser. No. 11/168,994 provides a safe delivery system of agriculturally active material is formed using an encapsulation process. A microencapsulated suspension of agriculturally active material includes an encapsulating agent formed from a graft copolymer of starch and at least one vinyl monomer. The polymers used in the encapsulation process as described in the co-pending application are biodegradable polymer. Starch is hydrophilic and a partially water soluble polymer. In the encapsulation process, the starch provides a polymeric outer shell to an inner core material after grafting with the vinyl monomer. Furthermore, the microcapsules of agriculturally active material were produced without using any diisocyanate/isocyanate as a shell material. The microcapsules described in the co-pending application are preferably more suitable for the quick or fast release of the active material and is more suitable for foliar application.
Slow-release microcapsules provide longer term efficacy as the encapsulated material release the active ingredient over a period of time. Moreover, slow-release microcapsules containing the agriculturally active ingredients may be useful in agricultural and non-agricultural applications.